In automatic contact lens production processes, in particular for the production of contact lenses which are to be produced cost-effectively in large numbers, the contact lenses are preferably produced by what is known as the mold or full-mold process. In these processes, the lenses are produced in their final shape between two casting mold halves (molds), so that neither subsequent machining of the surfaces of the lenses nor machining of the edge is required. Mold processes are described, for example, in WO-A-87/04390 or in the EP-A 0 367 513.
In these known mold processes, the geometry of the contact lens to be produced is established by the cavity defined between the casting mold halves. The edge of the contact lens is likewise formed by the casting mold, usually comprising two casting mold halves. The geometry of the edge is established by the contour of the two casting mold halves in the region in which they contact each other.
To produce a contact lens, firstly a specific amount of the flowable starting material is introduced into the female casting mold half. After that, the mold is closed by placing on the male casting mold half. It is customary for a little more starting material than is necessary to be fed in, so that, when the casting mold is closed, the excess amount is displaced into an overflow space adjoining the cavity on the outside. The subsequent polymerization or crosslinkage of the starting material takes place by irradiation with UV light or by heat exposure or some other non-thermal method. In this case, both the starting material in the cavity and excess material in the overflow space are cured. To obtain flawless separation of the contact lens from the excess material, good sealing and displacement of the excess material must be achieved in the contact zone of the two casting mold halves. Only in this way flawless contact lens edges can be obtained.
The contact lenses produced in this way are flexible, soft moldings with a water content of typically over 60% by weight. After its completion, the contact lens also undergoes verifying test measurements, is then packed and subjected to high-temperature sterilization (for example at 121° C.) in an autoclave.
Plastics, such as polypropylene for example, are used with preference as materials for these casting molds or casting mold halves. The molds are produced by injection molding and used only once. One reason for this is that the casting molds or casting mold halves are sometimes contaminated by the excess material, damaged when the contact lenses are detached or deformed irreversibly in partial regions when the casting mold is sealed. In particular because of the requirements demanded of the quality of the contact lens edge, the casting molds are used only once, since a certain deformation of the casting molds in their edge region cannot be ruled out with certainty.
U.S. Pat. No. 5,508,317 describes a novel contact lens material which represents an important improvement of the polymerizable starting materials for the production of contact lenses. The patent discloses a water-soluble composition of a prepolymer, which is filled into the cavity and subsequently crosslinked photochemically. The prepolymer carries a plurality of crosslinkable groups, and the crosslinkage process is distinguished by high speed, so that a finished contact lens of optical quality can be produced within a few seconds, without subsequent extraction or secondary finishing steps being required. Contact lenses can be produced at considerably lower costs by the improved starting material presented in the patent, so that as a result the production of disposable contact lenses is made possible.
EP-A-0 637 490 describes a method by which a further improvement of the production process for contact lenses can be achieved with the prepolymer described in U.S. Pat. No. 5,508,317. The starting material (prepolymer) is in this case filled into a casting mold comprising two casting mold halves, the two casting mold halves not being in contact with each other, but instead a thin, annular gap being arranged between them. The gap is in communication with the cavity, so that excess starting material (prepolymer) can flow away into the gap. The crosslinkage of the starting material takes place by irradiation, in particular with UV light, a delimitation of the irradiation to the mold cavity being achieved for example by a chromium mask. Consequently, only the material located in the cavity is crosslinked, so that a high degree of reproducibility of the edge shapes of the contact lens can be achieved without completely closing the two casting mold halves.
In this method, reusable quartz glass molds are used in practice, since, on account of the water-soluble starting material, they can be quickly and effectively cleaned of the uncrosslinked prepolymer and other residues and dried after a contact lens has been produced. Furthermore, quartz glass is distinguished by very good UV permeability and is very hard and resistant. In addition, quartz is an established substance as a carrier material for chromium masks in UV lithography.
In EP-A-0 941 829, use is made of the principles explained above for the efficient production of contact lenses. For this purpose, a molding tool which has two tool halves is proposed in EP-A-0 941 829. Each tool half has a number of receptacles, into which respectively complementary casting mold halves are inserted, to be precise for example female casting mold halves into one tool half and the associated male casting mold halves into the other tool half. The casting mold halves are clamped with high precision in the receptacles and, when the tool halves are closed, the tool halves and consequently the casting mold halves are positioned precisely in relation to one another.
Such a casting mold half clamped into a receptacle in each case comprises a mount, for example made of brass, which has a channel and a wall surrounding this channel. Provided at one end of the mount is an insert, which is accommodated by the mount and has a shaping surface for the shaping of the front or rear surface of the contact lens to be produced. The insert is produced, for example, from the quartz glass mentioned above, the previously already mentioned chromium mask being applied for example to the insert of the male casting mold half.
The chromium mask applied to the quartz glass has a layer thickness of about 0.3 μm and is preferably produced by processes known in photolithography or UV lithography. However, it is found that the chromium masks produced in this way often do not have the service lives required for an automatic production process and exhibit damage at an early time due to detachment of the chromium layer. Since, however, the inserts made of quartz glass with the chromium layer applied to them are very expensive to produce, the service lives of the casting molds should be very long to allow a cost-effective process to be ensured.